1. Field of the Invention
The present invention relates to an injection molding machine such as an in-line screw type injection molding machine and a pre-plasticization type injection molding machine and particularly to an injection molding machine in which completion of reduction of resin pressure in suck back control and completion of reduction of resin pressure by reverse rotation of a screw can be detected with accuracy.
2. Description of the Related Art
Injection molding can be divided into a process of injecting resin into a metal mold by moving a screw forward and forming a molded article by pressure holding and a process of moving the screw backward while rotating it at predetermined back pressure to meter a predetermined amount of resin into a cylinder. Because resin pressure has been developed after the end of the metering process, there is a process (referred to as a suck back process) of moving the screw backward by a predetermined amount to reduce pressure of the resin in the cylinder to prevent leakage of the resin from a nozzle portion when the metal mold opens.
A purpose of the suck back process is to reduce the pressure in the cylinder as described above. If the screw moves backward excessively, air may be drawn in from the nozzle and, as a result, a problem of air bubbles mixed into the molded article may occur. Therefore, as an optimum amount (suck back amount) by which the screw is moved backward, it is preferable to choose such a position that the pressure in the cylinder is sufficiently reduced and that the screw does not move backward excessively.
For this purpose, conventionally, a method of setting an optimum screw movement amount while monitoring a relationship between the resin pressure and a position of the screw indicated on a screen of an injection molding machine is employed. Furthermore, a method of automatically moving the screw backward until the resin pressure reaches a predetermined value (normally zero atmospheric pressure in terms of gage pressure) is disclosed in the following Patent Documents. With regard to gage pressure, when atmospheric pressure or surrounding pressure is zero, pressure higher than that is referred to as positive gage pressure and pressure lower than that is referred to as negative gage pressure. Hereafter, pressure in this specification is gage pressure.
Japanese Utility Model Application Laid-open No. 63-124115
Japanese Patent Application Laid-open No. 1-148526
Japanese Patent Application Laid-open No. 2-204017
Japanese Utility Model Application Laid-open No. 4-37322
Japanese Patent Application Laid-open No. 6-71705
However, a zero point offset may occur in a sensor for detecting the resin pressure as the sensor is used successively. Therefore, actual resin pressure may not have been reduced completely though sensor pressure is zero or the actual resin pressure may have been reduced completely though the sensor pressure is not zero. Moreover, to calibrate the sensor to correct the zero point, continuous molding normally need be stopped temporarily, which makes it difficult to constantly keep an accurate zero point of the sensor.
In general, for detection of the resin pressure, a method of detecting a load on the screw from the resin by using a load converter (load cell) is employed in many cases. However, the load that the screw receives includes not only reaction force from compressed molten resin in front of the screw but also viscosity resistance of semi-molten resin at a base of the screw. Therefore, the pressure detected by using the load converter (load cell) includes error due to the viscosity resistance of the resin and does not agree with the resin pressure even after accurate calibration.
As described above, it is difficult to accurately judge the completion of reduction of the pressure by the method of judging the completion of reduction of the pressure by comparing the value of the pressure sensor and the predetermined value (normally, zero atmospheric pressure).